Bark at first smooth, green, whitish, or orange, becoming gray and fissured on older stems. Twigs moderate, terete or somewhat angled; pith small, 6-angled, continuous or nearly so, brown. Buds moderate, ovoid or elongated, sessile, solitary, with several exposed scales, the lowest placed centrally over the leaf-scar. Leaf-scars alternate, broadly crescent-shaped to triangular, somewhat 3-iobed, large; bundle-traces 3, or sometimes compound.
Lateral buds with more than 4 exposed scales
b. Twigs and buds shiny or varnished, mostly glabrous
b. Twigs and buds gray, dull, buds silky or tomentose
c. Twigs glabrous
c. Twigs hairy or tomentose
Lateral buds with about 4 exposed scales
b. Terminal buds mostly 12 mm. or more long.
c. Twigs reddish-brown;buds quite resinous, very fragrant when crushed
d. Native tree of Canada and northern United States
d. Cultivated tree of horticultural origin
c. Twigs yellow to yellowish brown;buds slightly resinous
d. Buds widest at the base tapering toward the apex; planted tree
Fig. 29. Populus tremuloides.
Fig. 30. Populus grandidentata.
Fig. 31. Populus alba.
Fig. 32. Populus deltoides.
Fig. 33. Populus nigra italica.
Fig. 34. Populus gileaderisis.
d. Buds fusiform (widest near the middle, tapering both ways);native tree
Terminal buds mostly less than 12 mm. long
c. Buds short and broad; native swamp tree
c. Buds slender, tapering; form of tree tall and columnar; introduced
P. nigra, var. italica
1. P. tremuloides Michx. Quaking Aspen. A slender tree reaching a height of about 35m. and a trunk diameter of 1 m.; bark smooth, light green to white; twigs slender, shiny, reddish-brown; buds conical, brown to black, with 6 or 7 glabrous or cili-ate, somewhat gummy scales; leaf-scars conspicuous, lunate; stipule-scars linear, blackish. Dry or moist woods, Labrador to Alaska, south to West Virginia, Illinois, Iowa, New Mexico, Arizona, and Mexico; the most widely distributed tree in North America (Fig. 29).
2. P. grandidentata Michx. Bigtooth Aspen. A tree 10-25 m. high, 3-6 dm. in diameter; bark smooth, grayish-green;twigs grayish or yellowish-brown, rather thick, glabrous, dull; buds gray, puberulous, with about 6 or 7 visible scales; flowers in catkins, appearing in March. Dry woods and fields, Quebec to Minnesota, south to North Carolina and Missouri.(Fig. 30).
3. P. alba L. White Poplar. A large tree attaining a height of 30 m. and a trunk diameter of 2 m.; bark smooth, light gray;twigs and buds white-tomentose. Introduced from Europe, spreading and naturalized, often too abundant (Fig. 31).
4. P. deltoides Marsh. Eastern Cottonwood. A large tree, the greatest of the eastern poplars, sometimes 40 m. tall and with a diameter of 2 m. ; bark rough on old trees, grayish-green; twigs stout, angular, yellowish-brown, glabrous; lenticels large; terminal buds glabrous, lustrous brown, resinous, with 6 or 7 visible scales, the lateral buds usually smaller, divergent; leaf-scars large, lunate, elevated; stipule-scars dark, conspicuous. River banks and bottomlands, Quebec to Alberta, south to Florida and Texas (Fig. 32).
5. P. nigra L. var. italic a Muenchh. Lombardy Poplar. Tree to 30 m.high, the branches closely ascending, forming a narrow columnar crown; twigs glabrous, orange, changing to gray; buds slender, glabrous. Introduced from Europe as an ornamental tree, spreading by sprouts; usually staminate. A striking tree because of its formal columnar habit, often planted along avenues (Fig. 33).
6. P. balsamifera L. Balsam Poplar. Tacamahac. (P. tacamahacca Mill.). A tree to 30 m. high, the trunk up to 2 m. in diameter; bark on young stems greenish or reddish-brown, on older trunks becoming gray or grayish-black, divided into scaly or shaggy ridges; buds large, heavily coated with yellow balsam-scented resin; branchlets lustrous, terete. River-banks and rich soil, Labrador to Alaska, south to New York, Michigan, and Colorado.
7. P. gileadensis Rouleau.Balm of Gilead. Tree to 30 m. with stout spreading branches; twigs brown, pubescent; buds large, viscid. Horticultural in origin; only the pistillate tree is known, spreading by sprouts and cuttings (Fig. 34).
8. P. eugenei Saint-Simon. Carolina Poplar. A tree to 30m. high of pyramidal habit; twigs green, gray or buff; buds small, viscid, tapering from base to apex. Horticultural in origin; only the staminate tree is known, spreading from sprouts and cuttings; formerly much planted as a street tree but now in disfavor because the roots clog sewers (Fig. 35).
9. P. heterophylla L. Swamp Cottonwood. A tree up to 30 m. high; bark furrowed, in narrow plates, somewhat scaly; branch-lets whitish-tomentose, becoming glabrate and lustrous; buds 1- 1. 5 cm. long, canescent-tomentose. Swamps and bottomlands, mostly in the coastal plain, from Louisiana and Florida north to New York, inland about the Great Lakes.
Fig. 35. Populus eugenei.